In the years 2002-2005, a four-year long field experiment was conducted with three grass species in sole cropping. The study aimed at evaluating the effect of irrigation and differential nitrogen fertilization on phosphorus content in three swaths of grasses. The study was made in three-factor design test where experimental factors involved: (A) water treatment: objects both irrigated (D) and not-irrigated (ND) - representing natural soil moisture conditions; (B) grass species: Italian ryegrass (Lolium multiflorum L. Mitos); hybrid ryegrass (Lolium hybridum L., Trogres); orchard grass (Dactylis glomerata L., Krysta) and (C) differential nitrogen fertilization (0, 60, 120 and 180 kg N•ha-1). The highest levels of phosphorus were determined in orchard grass. The content of phosphorus in all grass species increased in subsequently harvested swaths this trend resulted from decreasing yields. A significant increase in P content in the grass biomass was found in response to irrigation.